2847
Macromolecules 1992,25, 2847-2853
Spectral Editing in Solid-state 13C MAS NMR of Elastomers Niels Chr. Nielsen,' Henrik Bildsm, and Hans J. Jakobsen Department of Chemistry, University of Aarhus, DK-8000Aarhus C, Denmark Received October 31, 1991; Revised Manuscript Received January 22, 1992
ABSTRACT Spectral editing techniques combined with magic-angle spinning (MAS) are introduced for assigment of the individual resonances in solid-state 13CNMR spectra for the amorphous (mobile) phases of elastomers. The proposed MAS SEMUT and MAS ESCORT pulse techniques allow unambiquous classification of the 13C resonances according to the number of protons attached to the contiguous carbon. The methods are demonstrated experimentally for samples of natural rubber (cis-1,4-polyisoprene),polychloroprene (neoprene)rubber, ethylene-propylenerubber, styrene-butadiene rubber, tire rubber, and kariten (mainly trans-1,4-polyisoprene).
Introduction During the past years pulse techniques for spectral editing,'+ i.e., separation of 13C resonances according to the number of attached protons (multiplicity determination), have gained considerable popularity as a tool for assignment of liquid-state 13C NMR spectra. This includes, for example, spectral editing of 13CNMR spectra for elastomers in solution.+12 Spectral editing or multiplicity determination using the DEPT pulse sequencea4 has also been performed using static12J3and magic-angle spinning (MAW4 NMR for solid elastomers heated to about 60 OC in order to increase the chain mobility and thereby their isotropic ("liquidlike") behavior. Recently, the techniques have been extended to include solid-state cross-polarization (CP) MAS NMR experiments15 for spectral editing of solids with relatively high intrinsic molecular mobility. For solid samples labeling of the 13CH, (n = 0, 1,2, and 3) multiplets during a conventional spinecho editing sequence requires averaging of interactions from dipolar coupling and 13Cchemical shielding anisotropy. This can be accomplished using a combination of MAS, rotor synchronization, homonuclear multipulse decoupling, and high-power 'H decoupling (HPD).16J7 For rigid solids, however, the experimental requirements for sufficient averaging to resolve the J multiplets may represent a difficult task.18 For such samples some degree of spectral editing may be achieved using techniques exploiting heteronuclear dipolar interactions for multiplet labeling such as the recently introduced crosspolarization/depolarization(CP/CPD)19320 and asynchronous MASSLF21pulse sequences. It is well-established that for the amorphous phase of bulk elastomeric compounds the dipolar interactions are substantially averaged by rapid reorientation of the molecular chain ~ e g m e n t s . ~In~ -fact, ~ ~ near isotropic conditions can often be obtained using MAS thereby rendering elastomers well-suited for a variety of liquidstate type NMR experiments such as spectral editing techniques. In this paper we present what to the best of our knowledge is the first MAS l3C spectral editing experiments applied to elastomers in the solid state. Edited spectra, obtained using the SEMUT5.6 and ESCORT' pulse techniques combined with MAS and HPD, are reported for various industrially important elastomers at ambient temperature. Experimental Section
ethylene-propylene rubber (EPR), styrene-butadiene rubber (SBR), tire rubber, and kariten. Kariten is a crude mixture of cis- and trans-1,4-polyisoprene (in a ratio of approximately 1:7) obtained in large quantities as a distillation residue in production of vegetable oil. The solid elastomers were used without further purification and were cut into small pieces before packing into the MAS rotor. A dissolved sample of kariten, approximately 3% w/w in CDC13, was also examined. B. Instrumentation. High-resolutionl3C NMRexperimenta were performed using a Varian XL-300(7.05-T) spectrometer operating at 75.43 MHz for 13C. The solid-state NMR experiments were carried out a t ambient probe temperature (ca. 25 "C) using a home-built high-speed spinning CP/ MAS probez6with 7-mm-0.d. Si3N4 rotors (220-pL sample volume) capable of handling imbalance from imperfect packing of the rubber pieces. Spinning speeds (u,) in the range of 3.5-4.5 kHz were employed. The 13Cchannel radio-frequency field strength was typically 42 kHz, corresponding to a90" pulse width tw(13C)= 6.0 ps, and the lH radio-frequency field strength used for editing pulses and HPD was 33 kHz, corresponding to t&H) = 7.6 ps. 13Cchemical shifts are in ppm relative to an external sample of tetramethylsilane (TMS). C. Experimental Techniques. The two spectral editing techniques employed in this work have been analyzed in detail in separate papers, where rigid and liquidk7 versions of the experiments are considered. In this section we briefly review some of the main features of the techniques applicable to the amorphouslmobile phases of elastomers. Pulse schemes for the MAS SEMUT and MAS ESCORT experiments are shown in parts a and b of Figure 1, respectively. The sequences are identical to their liquid-state analogues apart from (i) MAS is applied to average dipolar interactions and chemical shielding anisotropy, (ii) detection is performed under HPD, and (iii) the lH decoupleris turned off during the relaxation delay to protect the radio-frequency circuitry. In both experiments transverse 13C magnetization, created by an initial SOo (13C)pulse, evolves under the influence of heteronuclear scalar J couplings during a spin-echo editing fragment. The efficiency of this evolution can be controlled by varying the length of the 0-editing pulse in MAS SEMUT (Figure la) or the length of the decoupler-off period 7 in MAS ESCORT (Figure Ib), while maintaining a constant length of the overall spin-echo periods. Discrimination of 13Csignals from the various one-bond coupled CH, (n = 0,1,2, and 3) spin systems is achieved by appropriate linear combinationsof MAS SEMUT experiments recordedusing flip angles of 0 = Oo, 60°, 120°, and 180' with relative number of scans l:2:2:15 or MAS ESCORT experiments recorded using 0 = O', 60°,120°, 180°, 240°, and 300' ( 8 = 180'57) with relative number of scans l:1041:2:2.7 Under ideal conditions complete editing into subspectra, containing only one multiplicity, requires a combination of spectra from these subexperiments using the In practice the Coefficients may coefficients listed in Table differ somewhat from the ideal values, and in this study we employed a least-squares procedure for an optimum linear combination of the subexperiments. L5l7
A. Samples. Six elastomers of commercial origin have been examined in this work. The samples are natural rubber (NR, cis-1,4-polyisoprene),polychloroprene (neoprene) rubber (CR),
0024-929719212225-2847$03.00/0 0 1992 American Chemical Society
Macromolecules, Vol. 25, No. 11, 1992
2848 Nielsen et al.
Figure 1. Pulse sequences (a) MAS SEMUT and (b) MAS ESCORT employed for subspectral editing of 13CNMR spectra for elastomers. The 7 delays in MAS SEMUT should be chosen as 71 = {2[Jmin 0.146(Jm, - Jmin)])-land 73 = {2[Jm, - 0.146 (J,,,,- Jmin)])-l to reduce J cros~-talk.~~6 The actual range for the heteronuclear (13C-lH) J coupling nstants is given by Jmin J < Jm=.The fixed delay 70in MAS %CORT should be adjusted to 70 = 5(35)-l for complete editing into individual CH, (n = 0, 1, 2, and 3) subspectra, whereas 70 = (4-lshould be used for editing into n even (C, CHz) and n odd (CH, CH3) subspectra (J = (Jmin + J m u ) 1 2 ) . For MAS ESCORT the "editing angle" is defined as 0 = 180"Jr.
+
Table I Ideal Coefficients for a Linear Combination of MAS SEMUT and MAS ESCORT Subexperiments Leading to Complete Spectral EditingSJ
f
C
1
100
1LO
60
20
ppm
Figure 2. 13CMAS ESCORT edited spectra of NR (J= 135 Hz, 70 = 15 ms, v, = 3485 Hz, and 4-5 relaxation delay). (a) The 0 = 0" (7= 0) subexperiment (32 scans) with "unity" intensity for all carbons. (b-e) Complete edited subspectra obtained from a linear combination of 0 = 18O"Jr = n60" (n = 1, ...,5) subexperiments (see text). (0 Quaternary carbon spectrum resulting from the special quaternary-only version' of the MAS ESCORT experiment.
Ob
multiplicity (0') (180') (6Oo),(30O0) (12O0},(24O0} -1110 315 C -1110 215 215 113 CH -1118 419 -419 1/18 318 CH2 114 114 -114 -114 116 -116 114 CH3 -113 113 The factors reflect ideal signal intensities observed in the completely edited subspectra.6J * The 0 = 300' and 0 = 240' subexperimenta, which apply only for MAS ESCORT, use the same coefficientsas the 0 = 60' and 0 = 120" subexperiments,respectively. An intrinsic feature of the Figure 1 spectral editing sequences is that the highly mobile carbons associated with the amorphous phase of the elastomers are selectively probed. Signals from rigid bound protonated carbons undergo efficient dipolar dephasing throughout the long periods without HPD of the spin-echo editing fragments.26 In contrast, less mobile (e.g., crystalline) phases may be observed selectively using excitation via CP,27 and possibly the CH, multiplicities may be labeled using MAS SEMUT with homonuclear multipulse decoupling and rotor synchronization of the spin-echo periods.15 For the simple MAS SEMUT sequence all subexperiments contain a decoupler-off period of 6.5-8.5 ms, which usually leads to efficient dephasing for less mobile parts of the sample. For MAS ESCORT the spinecho period is usually on the order of 2~~ = 22-28 ms with a variable decoupler-off period (7) leading to efficient dipolar dephasing (rigid parts) for all subexperiments except for the 7 = 0 (0 = 0") experiment. Anyway, the very long spin-echoperiod for this experiment usually causes suppression of "rigid" carbons due to rapid transverse Tz relaxation. One point of concern when applying l3C spectral editing techniques to solid elastomers is their performance for carbons with short transverse relaxation times (T2).Another point is the influence of radio-frequency field inhomogeneity. In cases of short Tis,the MAS SEMUT sequencewith a total length (ideally) of 1 / J is preferred over the alternative DEPT3.4 (length 3/(25)) and MAS ESCORT (length 10/(3&) techniques. MAS ESCORT is the technique of choice in the case of longer Tis because the editing fragment of this sequence is not influenced by radiofrequency inhomogeneity.
Results and Discussion T h e MAS SEMUT and MAS ESCORT editing techniques are demonstrated experimentally for NR, CR, EPR,
I'
'
)!-,-)m
J(
A
d 1LO
~
120 ppm
LO
CH*CH, 20
Figure 3. 13C MAS NMR spectra of CR. (a) Standard 13C MAS spectrum (v, = 4150 Hz, 896 scans, 4-5 relaxation delay). (b-d) MAS SEMUT spectra showing (b) all CH, resonances and (c and d) partially edited spectra containing CH, resonances with (c)even and (d) odd parity of n. The c and d spectra were obtained as the sum and the difference,respectively,of 0 = 0" and 0 = 180" experiments (v, = 4150 Hz, 71 = 4.02 ms, 7 3 = 3.43 ms, 2400scans, 6-s relaxation delay).
SBR,tire rubber, and kariten in Figures 2-9. In addition, for most of the samples a standard H P D 13CMAS spectrum was recorded t o evaluate influences from relaxation and dipolar dephasing throughout the editing sequences. l3C chemical shifts and assignments obtained from the spectra are summarized in Table 11. A. Natural Rubber ( N R ; cis-1,4-Polyisoprene). Figure 2 shows t h e edited MAS ESCORT 13Cspectra for NR obtained using the Figure l b pulse sequence and ur = 3485 Hz. The "standard" CH, spectrum in Figure 2a is that of t h e 6 = 0" (7 = 0) MAS ESCORT experiment, while t h e fully edited CH3, CH2, CH, and C subspectra are shown in Figure 2b-e. As an alternative quaternary carbon (C) spectrum, the spectrum in Figure 2f was obtained using a special quaternary-only version' of t h e MAS ESCORT
Mucromolecules, Vol. 25, No. 11, 1992
Spectral Editing of Elastomers 2849
c
i C
CH,
1
d
180°
P
CH+CH? I " "
50
LO
30
20
PPm
Figure 4. 13C MAS NMR spectra of EPR. (a) Standard 13C MAS spectrum (v, = 3510 Hz, 512 scans,4-s relaxation delay) (b) t9 = 0' MAS SEMUT spectrum (v, = 3510 Hz, T~ = 4.09 ms, T~ = 3.76 ms, 4096 scans, 4-s relaxation delay). (c and d) Partially edited CH2 (c) and CH + CH3 (d)MAS SEMUT subspectra.The minor negative or dispersive signals in d are ascribed to J crosstalka5
experiment. The spectra demonstrate that spectral editing provides unambiguous assignment of the C, CH, and CH3 resonances to C,, Cg, and C,, respectively, for NR (stuct u r d formula is indicated in Figure 2). The C, and Ca resonances in the CH2 subspectrum may be distinguished according to standard additivity rules.28 The result of the spectral editing is consistent with the assignment reported for bulk polyisoprenes in an early study by Duch and Grant.23 When combined with MAS, rapid reorientation of the chain segments makes spectral editing based on heteronuclear J modulation during the 7 period of the MAS ESCORT experiment a simple task as demonstrated here for NR at room temperature. B. Polychloroprene (Neoprene)Rubber (CR).Proton-decoupled 13CMAS NMRspectraof CR are illustrated in Figure 3. The 13CH, (all-carbon) spectra in parts a and b of Figure 3 are those of a standard 13CMAS experiment and the corresponding 8 = Oo MAS SEMUT experiment, respectively. The partially edited 13C MAS SEMUT spectra shown in parts c and d of Figure 3 were edited according to CH, fragments with n even (C and CHZ)and n odd (CH and CH3), respectively, using the 8 = Oo and 8 = 180° subexperiments. Inspection of these subspectra along with standard additivity rulesz8enables assignment of the four major resonances to the C, (135.6 ppm), Co (124.7 ppm), C, (39.0 ppm), and Ca (27.5 ppm) carbons of spuren truns-1,4-CR in accordance with the assignments given by Coleman et aLZ9(the Greek letters refer to the structural formula). Here the term pure means that the truns-1,4-CR monomeric units are linked exclusively in a head-to-tail fashion. The spectrum also displays a number of weaker, more or less resolved signalsand shoulders which indicate structural irregularities throughout the sample. These resonances most likely result from head-to-head and tail-to-tail linkages of truns-1,4-CR or from polym-
1LO
120
45 ppm 25
25
L5
Figure 5. 13C MAS NMR spectra of SBR. (a) Standard 13C MAS spectrum (v, = 4340 Hz, 4096 scans, 4-s relaxation delay). (b and c) 13C MAS SEMUT spectra using (b) t9 = 0' and (c) B = 180' (same parameters as in a). The spectra in the right column are identical to those in the center column but with a vertical scale expansion by a factor of 5.
C
P
d
I
A
h
hi.
CH2
CH
Figure 6. 13CMAS NMR spectra of a tire rubber. (a) Standard W MAS spectrum (v, = 3500 Hz,524 scans, 4-s relaxationdelay), (b) 0 = 0' (T = 0) MAS ESCORT spectrum. (ef)Complete 13CH, (n = 3,2,1, and 0) edited MAS ESCORT subspectra. The MAS ESCORT experimentsemployed v, = 3510 Hz, TO = 15 ma, 2-s relaxation delay, and 6720 scans distributed between B = n60' (n = 0, ...,5) subexperimentsin a ratio of 1:102:1:2:2. The symbols A, A, and 0 refer to cis-1,4, trans-1,4, and 1,2units of polybutadiene, whereas t refers to styrene units in SBR. erization with cis-1,4-, 1,2-, or 3,4-CR units. For simplicity, we denote the tail-head configuration, -CH~CtCl==Ch-
Macromolecules, Vol. 25, No. 11, 1992
2860 Nielsen et al.
I
I
Figure 9. 13CNMR spectrum for a solution of approximately 3 % w/w kariten in CDCl3. 256 scans were accumulated with a relaxation time of 4 s. The symbols follow the definitionsgiven for Figure 6.
:
p
11
d
/
1
A&
P
135
125 ppm
LO
30
20
10
Figure 8. 13C MAS SEMUT spectra of kariten. (a) 0 = 0'
spectrum showing all WH, resonances with positive intensity. (b)0 = 180°spectrum with up/down intensitiesfor the CH, groups with even/odd parity of n. (c and d) Partially edited spectra containing the C + CH2 (c) and CH + CH3 (d) resonances and obtained as the sum and difference, respectively, of the spectra ina and b. The experimentalparametersare given in the caption of Figure 7.
HCH2- (see Coleman et aL29,of the tram-l,l-CR unit by N, the inverted unit (head-tail) by I, and the cis-1,4-CR unit by C. Using this shorthand notation in combination with the liquid-state spectral assignments reported by Coleman et al.,Bit is possible to identify moat of the signals/ shoulders listed in Table I1 to the central unit of various triad sequences (e.g., NNN or "1). From the assignment in Table I1 it is concluded that the majority of these
resonances are caused by tram-1,4-CR units in different environments. The weak signal observed in the C + CH2 spectrum (Figure 3c) at 32.8 ppm can be assigned to the methylene C, carbon of a cis-1,4-CR unit linked to two N trans-1,4-CR units in a head-to-tail configuration. The presence of a small amount of the cis-1,4-CR isomer (approximately 5-10%) is also supported by the C, (quaternary) and Cb signals of this monomer observed as shoulders at 134.4 and 127.1 ppm in the C + CH2 and CH + CH3spectra, respectively. Within the limit of the signalto-noise ratio the spectra do not show signals from 1,2- or 3,4-CR units. In addition to the signals assigned above, the spectra contain a few weak resonances (Table 11) of which the CH2 signal at 30.5 ppm and the CH signal at 32.6 ppm should be particularly noted. The latter is unambiguously discriminated from the 32.8 ppm C, carbon of cis-1,4-CR by the editing experiment. The distribution of the different molecular configurations of the CR monomeric units estimated from the spectra is in accordance with typical compositions of commercial neoprene rubber.30 It should be noted, however, that the degree of crystallinity in the sample depends on the amount of t r ~ m - 1 , 4 - C Rand ~ ~ that the MAS SEMUT spectra solely contain contributions from the noncrystalline part9 of the sample. The existence of carbons residing in less mobile (crystalline) environments becomes plausible by comparison of the spectra in parts a and b of Figure 3 which show a noticeable attenuation of signals from protonated carbons in the MAS SEMUT spectra. This observation may be ascribed to dipolar dephasing (or T2 relaxation) for the crystalline regions during the spin-echo period. This conclusion is supported by CP/MAS experimenta (not shown)for oursample which show considerable polarization transfer and thereby the presence of dipolar-coupled carbons. Therefore, with an unknown fraction of the sample in an immobile/cryatalline phase, accurate measurement of the cis-1,CCRversustram1,4-CRratio is not possible from the present experiments. C. Ethylene-Propylene Rubber (EPR).Figure 4 illustrates the standard l3C MAS (Figure 4a) and MAS SEMUT (Figure 4b-d) spectra of EPR. The standard MAS and 0 = Oo MAS SEMUT spectra both include carbon resonances from the highly mobile (amorphous) phase; however, some broad components (less mobile part) of the resonances in Figure 4a appear with reduced intensity in the MAS SEMUT spectrum (Figure 4b). The partially edited MAS SEMUT spectra resulting from the 0 = ' 0 and 180° subexperiments discriminate between CH2 (Figure 4c) and CH + CH3 (Figure 4d) resonances. Careful inspection of the spectra in parta a and b of Figure 4 permits identification of 14 more or less well-resolved resonances
Macromolecules, Vol. 25, No. 11, 1992
Spectral Editing of Elastomers 2851
Table I1 Chemical Shifts, Multiplicities, and Assignment of '42 Resonances for the Six Solid Elastomers. CR
NMR b
135.4' 125.S 33.0' 27.3' 24.19
assignmentb
b
assignmentbvc
EPR 6
Ca-cis-1,41 135.6' C r ~ i ~ - 1 , 4 I134.6' C,-ci~-l,41 134.4' C&-l,4I 134.2' C,-ci~-l,4I 127.1d 126.3d 125.4d 124.7d
C0-NNN-1,4C Cm-NNI-1,4C Cm-NCN-1,4C Ca-NIN-1,4C CrNCN-l,4C CrNIN-1,IC C,q-NNI-l,IC CrNNN-1,IC
46.1' 39.1' 38.2' 35.4' 33.7d 31.1d 30.8' 28.5d
39.0' 38.6' 32.8' 32.6d 30.5' 28.4' 28.2' 27.5' 23.6' 20.5 15.0
CT-NNN-1,4C CT-NIN-1,4C C,-NCN-l,IC
28.2' 25.5' 23.7'
assignmentd S .. S ,, Sa6
S@
Tw,T y a , T66 Tp,, Tg6 sy,,
Sy6, S66
T,98 s87, sg6
s#g
Ca-NNI,NIN-l,4C Ca-NNI,NIN-l,4C Ca-NNN,NCN-l,4C
SBR 6
145.7' 143.W 131.9 130.9 130.1d 128.9 128.3d 126.6d 115.1' 48.0' 46.4d 44.1d 41.9' 41.1' 39.0' 36.5' 34.7' 33.5' 31.2' 30.9' 28.2' 25.8' 24.9s
tire rubber
6 S(Ph-CI) 145.7 143.1d u tS, tv, cS,cv 135.41 132.1 tt, tt, tc, ct 130.9 ct, tc, cc, cc 130.2d S(Ph-CZ,e) 128.9 S(Ph-Cs,s) S(Ph-Cd), St, 125.9 sc,vt, vc u 115.2 46.5 44.1d tS,cS 33.5' tu, cu 33.1' SS, uv 30.5' tS,SS, vS 28.2' tv, vu, Su 27.3' St, Sc, cv 26.0 ut, uc 24.2s tt, tt, ct, tc 23.5 St 20.4 vt 16.6s ct, tc, cc, cc 14.9 Sc, vc assignmente
kariten
6 Ph-Ci-S 135.7' CH-l,2B 135.4' 135.1' C.-CiS-1,41 131.1' Ca-tram-1,4B 130.5d Ca-cis-1,4B 130.3d Ph-Cz4-S 125.9 125.1d CrCiS-1,41 aseignmentbf
CHS-1,ZB CH-S CHz-l,2B Crtrans-1,4B C,-ciS-1,41 C&-1,4B C,-cis-1,41 C,-CiS-1,41
aesimmentb
121.4 40.6' 39.4' 34.7' 34.4' 33.0' 32.8' 30.6' 30.2' 30.0' 28.0' 27.5' 27.3' 25.6' 24.1s 23.6' 18.2s 16-64 14.9s
a In ppm relative to external TMS. Superscripts s, d, t, and q denote singlets (C), doublets (CH), triplets (CHz), and quartets (CH,), respectively, as determined by editing. C, (x = a,B, y, 6, e) refers to the structural formulas given in the figures. -1,41, -1,4C, -1,2B, -1,4B, and -Sdenote l,l-polyisoprene, 1,4-polychloroprene, 1,2-polybutadiene, 1,4-polybutadiene, and polystyrene, respectively; a-trans-1,41 refers to the a conformer of crystalline trans-1,4-polyisoprene. C,-XYZ (X, Y, Z = N, I, or C) refers to the C, carbon for the central unit of triad sequences (seetext). The assignments also apply for reversed sequences with N and I interchanged. The assignments refer to the interpretation and terminology of Cheng32 (see text). e The assignments to diad sequences (units in italics) use the terminology of Katritzky and Weiss.38 If diad not indicated, the assignment includes all diads. f Ph-C,-S (n = 1-6) refers to the phenyl C, carbon in SBR (see Figure 6).
and shoulders. Complete assignment of the relatively broad resonances associated with bulk EPR represents a considerable task due to the variety of possible ethylenepropylene connectivities, the propylene tacticity, and the presence of propylene inversion. The assignment of solution-state 13C NMR spectra for EPR has been addressed several times in the literature. In this work we find it relevant to compare the results of our edited spectra for the highly mobile phase of this elastomer with the assignment of the liquid-state spectra reported by Cheng et al.31-33For this purpose we employ the terminology of Cheng32with T, S, and P denoting CH (tertiary), CH2 (secondary), and CH3 (primary) groups, respectively, and the Greek letter subscripts refer to the number of bonds separating the considered 13C and the surrounding CH carbons. The assignment of the EPR resonances is given in Table 11. It is noted that the multiplicities of the 13CH, resonances obtained from the edited spectra are consistent with the assignment of Cheng;32the minor negative and dispersion-like signals in Figure 4d are the result of Jcrosstalk.5 D. Styrene-Butadiene Rubber (SBR). Figure 5 shows the standard 13C MAS (Figure 5a) and 13C MAS SEMUT (Figure 5b,c) spectra of SBR. The spectra in parts a and b of Figure 5 (MAS SEMUT, B = ) ' 0 display all 13CH, resonances with positive signal intensities, whereas positive and negative intensities in the MAS SEMUT 8 = 180' spectrum (Figure 5c) discriminate the signals according to n even and n odd, respectively. The high resolution observed in these MAS spectra (as compared to earlier reported solid-state NMR spectra of SBRS936)enables identification of a large number of minor resonances, resulting from a variety of different sequences of monomeric styrene and butadiene units. The resolution
of resonances and CH, multiplicity assignment (neven or odd) from the 8 = ' 0 and 8 = 180' MAS SEMUT experiments are consistent with the information obtained from a DEPT experiment (not shown) for a solution (CDCl3) of the same sample of SBR. Assignment of the I3C MAS spectrum (Table 11), following the MAS SEMUT information, may be completed using the assignment for solution-state spectra of SBR by Katritzky and Weiss.36 The assignment to diad sequences uses the nomenclature of ref 36 with c, t, v, and S denoting cis-l,Cbutadiene-, tram-l,Cbutadiene-, l,Zbutadiene-, and styrene-linked units, respectively, oriented according to the structural formulas in Figure 5. A couple of interesting features should be noted. First, MAS SEMUTnoticeably improves the spectral resolution as compared to standard MAS experiments. As for EPR, this may be ascribed to dipolar dephasing or T2 relaxation of less mobile components during the editing fragment. Second, on basis of relative signal intensities in the standard 13C MAS spectrum the composition of monomeric units in the sample is estimated to be 53% tram-l,Cbutadiene, 8 5% cis-1,4-butadiene, 12% 1,2-butadiene,and 27 % styrene (% w/w). These numbers are in accord with the results from the standard liquidstate 13C NMR spectrum for the SBR sample. E. Tire Rubber. I3C MAS NMR spectra of a tire rubber are shown in Figure 6 with the standard 13CMAS spectrum in Figure 6a, a 8 = ' 0 ( 7 = 0) MAS ESCORT spectrum displaying positive signals from all carbons in Figure 6b, and completely edited CH3, CH2, CH, and C MAS ESCORT subspectra in Figure 6c-f. Comparison of the spectra in parts a and b of Figure 6 indicates the presence of dipolar-coupled and/or fast-relaxing constituents from immobile (e.g., cross-linked) regions of the tire rubber since a number of broad resonances (e.g., at 26.0,
2862 Nielsen et al.
30.5, 46.5, 115.2, 128.8, and 145.7 ppm) are considerably attenuated in the MAS ESCORTspectra. The five intense resonances denoted with the symbol 0 in Figure 6a are immediately identified as cis-l,4-polyisoprene (the Greek letter symbols in the spectra refer to the corresponding carbons of the structural formula). Among other components in tire rubber, polybutadiene rubber (BR) and styrene-butadiene rubber (SBR) are usually the most predominant. The relevant structural formulas are shown above the spectra. The intense signals at 28.2 and 130.2 ppm are attributed to the CH2 and the diene CH carbons of cis-1,4-polybutadiene (symbol A),while the shoulders at 33.5 and 130.9 ppm are assigned to trans-1,4-polybutadiene (symbol A).23*37 Furthermore, the weak peaks observed at 44.1,115.2, and 143.1 ppm can be assigned to -CH2-, 4 H 2 , and =CH- constituents, respectively, of 1,2-polybutadiene (symbol 0). In addition to its contribution to the butadiene peaks, SBR manifests itself by weak resonances for styrene at 46.5,128.8, and 145.7 ppm (denoted by the symbol *). These broad signals, which are almost completely suppressed in the MAS ESCORT spectra, are assigned according to the spectrum of SBR in Figure 5. I t should be noted that quite large variations in the relative content of cis-1,4-polyisoprene, butadiene rubber, and SBR have been observed for different tires investigated in our laboratory. F. Kariten. As a final and intriguing example of spectral editing for solid elastomers Figure 7 presents I3C MAS SEMUT spectra of the amorphous phase for semicrystalline kariten (a crude mixture identified as mainly cis- and tram-1,4-polyisoprene and obtained as a residue from production of vegetable oil). Figure 7a shows the standard 13C MAS spectrum, Figure 7b the 8 = Oo MAS SEMUT spectrum, and Figure 7c-f fully edited CH3, CH2, CH, and C MAS SEMUT subspectra. The spectra demonstrate that (i) MAS SEMUT is very selective in probing/editing resonances only from the amorphous constituents of this mixture of polyisoprenes and (ii) assignment of the more crowded regions of the spectra is clearly facilitated by editing. From the spectra the two major components can immediately be identified as the cis and the trans isomers of 1,4-polyisoprene (structural formulas are shown above the spectra). The cis-1,4-polyisoprene resonances (symbol 0)are easily identified from the spectra of NR in Figure 2. Five of the remaining major resonances (symbolA)can be assigned to amorphous trum1,4-polyisoprene ( g u t t a - p e r ~ h a ) . ~It~appears r ~ ~ that resonances at 18.2,39.4,121.4, and 135.7 ppm in the standard 13C MAS spectrum are suppressed in the MAS SEMUT spectra. Using this observation and the assignment of Patterson and K ~ e n i gthese , ~ ~resonances may be assigned to the methyl C, (18.2 ppm), the methylene C, (39.4 ppm), and the olefinicCb (121.4 ppm) and C, (135.7 ppm) carbons for the monoclinic a conformer of crystalline tram-1,4polyisoprene (symbol A); we note that the Ca resonance at 27.0 ppm is not resolved in the Figure 7a spectrum. Apart from these intense signals a number of weak, but well-defined, resonances and shoulders are observed at 14.9ppm (CH3),23.6,25.6,28.0,30.0,30.2,30.6,32.8,34.4, and 34.7 ppm (CHd and 130.3 and 130.5ppm (CH) (Table 11) for our sample of kariten. Again it should be noted that weak (negative, positive, or dispersion-like) signals in the fully edited subspectra of Figure 7 may result from J cross-talk;6however, these artifacts are easily identified by comparison of the different spectra. Comparison of the MAS SEMUT spectra of kariten in parts c-f of Figure 7 with the 8 = ' 0 spectrum in Figure 7b reveals that complete editing is associated with a
Macromolecules, Vol. 25, No. 11, 1992 substantial loss in signal-to-noise (S/N).39 In numerous cases, including the polyisoprene samples, where the regions of C (CH) and CH2 (CH3)signalsare well-separated, partial editing into CH, subspectra with even and odd parity of n provides the necessary multiplicity information. Even/odd editing can in principle, as demonstrated for solid kariten in Figure 8, be performed without loss in S/N since the 8 = Oo and 8 = 180' spectra ideally contain full intensity for all carbons. It should be noted that the partially edited spectra also enable unambiguous multiplicity assignment of very weak signals, for example, in the region from 130.3 to 131.1 ppm, assignments that are most convincingly obtained from the 8 = 180' spectrum (Figure 8b). Information about the relative amounts of the cis and trans isomers of l,4-polyisoprene in our kariten sample and their phase distribution can beobtained by comparing with the standard '3C spectrum in Figure 9 of kariten dissolved in CDCl3. Measurement of relative peak intensities from the liquid-state spectrum (Figure 9) yields a total composition of ca. 15% cis-1,4- and 85% tram1,4-polyisoprene. The corresponding measurement for the MAS SEMUT B = ' 0 spectrum, which exclusivelydisplays signals from the amorphous phase, yields a composition of 35% cis-1,4- and 65% trans-1,4-polyisoprene. In combination, these numbers suggest that about 70% of the truns-1,4-polyisoprene occurs in a crystalline state at room temperature. This conclusion is supported by CP/ MAS experiments which show intense signals for the a and 0 conformers of crystalline trans-1,4-polyisoprene.
Conclusions This work demonstrates that spectral editing techniques represent a powerful tool for assignment of l3C resonances associated with the amorphous (mobile) phases of solid elastomers. The combined effect of (i) magic-angle spinning with proton decoupling during data acquisition and (ii) the molecular motions characteristic for the amorphous phase of elastomers allow "liquid-state type" NMR experiments such as spectral editing to be performed for these systems. The MAS SEMUT and MAS ESCORT pulse sequences presented here have been successfully applied to characterize the amorphous phases for a series of single-and multiple-component rubbers a t ambient temperature. Applications of the CP/MAS SEMUTtechnique and/or other spectral editing methods to the crystalline (immobile) phases of polymers are currently in progress. Acknowledgment. The use of the Varian XL-300 spectrometer, sponsored by the Danish Research Councils SNF and STVF, Carlsbergfondet, and Direktm Ib Henriksens fond, at the University of Aarhus NMR Laboratory is acknowledged. We thank the Aarhus University Research Foundation for equipment grants. Financial support from STVF and SNF is acknowledged. We thank several companies in Denmark for providing the samples studied in this research. References and Notes (1) Morris, G . A.;Freeman, R. J. Am. Chem. SOC.1979, 101, 760. (2) Burum, D. P.; Ernst, R. R. J. Magn. Reson. 1980, 39, 163. (3) Pegg, D. T.; Doddrell, D. M.; Bendall, M. R. J. Chem. Phys. 1982, 77, 2745. (4) Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. J. Magn. Reson. 1982,48, 323. (5) Bildase, H.; Dsnstrup, S.; Jakobsen, H. J.; Ssrensen, 0. W. J. Magn. Reson. 1983,53, 154. (6) Ssrensen, 0. W.; Dsnstrup, S.; Bildsse, H.; Jakobaen, H. J. J. Magn. Reson. 1983, 55, 347.
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